JPH0244581B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for regenerating a catalyst for removing nitrogen oxides from exhaust gas (hereinafter referred to as a denitration catalyst), and more specifically, the present invention relates to a method for regenerating a catalyst for removing nitrogen oxides from exhaust gas (hereinafter referred to as NOx) using ammonia as a reducing agent. Concerning playback methods. NOx in exhaust gas has recently attracted attention as one of the causative substances of photochemical smog, and various methods for its removal have been proposed. Among them, NOx is reduced to harmless nitrogen in the presence of a catalyst using ammonia as a reducing agent. The catalytic reduction method for this purpose has already reached the practical stage. The catalyst used here is generally a vanadium-titania catalyst, which exhibits excellent activity. Vanadium titania systems include combinations of V ₂ O ₅ and TiO ₂ and (V ₂ O ₅ + α component) and TiO ₂ , and the α component includes, for example, molybdenum oxide and tungsten oxide as described in JP-A-51-80696. and so on. Its composition is _TiO2 80~95wt%, _{V2O5 20} ~5wt%
%, but generally TiO ₂ 90-91wt%, V ₂ O ₅ 10
Many contain ~9wt%. The α components WO ₃ and MoO ₃ are blended in a form that replaces V ₂ O ₅ , such as 91 wt% TiO ₂ , 2 wt% V ₂ O ₅ , and 7 wt% WO ₃ . Furthermore, the shape of the denitrification catalyst is often a honeycomb shape as shown in FIG. Honeycomb catalysts can be roughly divided into coated type and solid type. The former is made by coating a honeycomb-shaped base material with a titania slurry containing a vanadium compound, dried and fired, while the latter is made by applying a titania slurry containing a vanadium compound to titania and water. It is made by adding, kneading and forming into a honeycomb shape, then drying and firing. (The arrow in Figure 1 indicates the direction of gas flow.) Denitrification catalysts are generally used in the temperature range of 350 to 450°C, but if the exhaust gas is dirty, the activity of the catalyst decreases with long-term use. A phenomenon is observed. Alkali metal compounds such as sodium and potassium, and earth metal compounds such as magnesium and calcium are well known as catalyst poisons that reduce catalyst activity, and these are contained in fuel oil and coal. In addition, blockage due to adhesion and accumulation of dust and a decrease in surface area are also factors that reduce activity. As mentioned above, there are various factors that cause the activity of the denitrification catalyst to decrease over time. Cleaning with an inorganic acid aqueous solution or spraying with steam (Japanese Patent Application Laid-Open No. 1973-
27091), after extracting a part of the vanadium component by contacting with an oxalic acid aqueous solution, impregnating and supporting the vanadium compound,
Activity recovery (regeneration) methods such as a firing method (Japanese Patent Laid-Open No. 10294/1983) have been proposed. However, although water-soluble catalyst poisons and dust are removed in the cleaning treatment, and some effects are observed, the active ingredients vanadium and α
Since some components (tungsten oxide, molybdenum oxide, etc.) are eluted, it is not possible to regenerate the catalyst to a level equivalent to the activity of the unused catalyst. In particular, in the method of washing with an aqueous oxalic acid solution, vanadium is forcibly eluted as vanadyl oxalate, and a considerable amount of the α component is also eluted, so naturally no improvement in activity can be expected. It is therefore necessary to reload the active ingredient after said washing. However, the above-mentioned coated catalyst cannot be regenerated properly by the conventional method of simply impregnating and supporting a vanadium compound after the washing, and can be successfully regenerated only by applying a titania slurry containing a vanadium compound or [vanadium compound + α component compound]. I discovered that. Ammonium metavanadate and vanadyl oxalate are commonly used as the vanadium compounds, and ammonium paratungstate and ammonium molybdate are commonly used as the α component compounds. Furthermore, as shown in Figure 1, exhaust gas passes through the holes in the honeycomb, so if the diameter of these holes becomes smaller, that is, if the thickness of the coating layer increases, the pressure loss will naturally increase, and the thickness of the coating layer will increase. Titania slurry cannot be applied blindly as it may cause cracking or peeling. The inventors have conducted various studies regarding this point, and
We have discovered that by controlling the viscosity of titania slurry within a certain preferable range, it is possible to regenerate a coated honeycomb catalyst with reduced activity into a catalyst with activity and pressure loss equivalent to that of an unused catalyst, and have completed the present invention. The present invention will be explained in detail below. To regenerate a coated catalyst according to the present invention, first, the catalyst whose activity has decreased is washed with water or a dilute aqueous solution of an inorganic acid such as sulfuric acid, nitric acid, and hydrochloric acid. Furthermore, washing with an oxalic acid aqueous solution is added as necessary. After that, titania slurry containing a vanadium compound or [vanadium compound + α component compound] (viscosity 0.3 to 2.0
A regenerated catalyst can be obtained by coating, drying, and baking. The composition of the vanadium compound and the α component compound in the titania slurry is such that the composition after firing is, for example, 91 wt% TiO ₂ ,
It is calculated and determined in advance so that V ₂ O ₅ 2wt% WO ₃ 7wt%. As a result of being used as a denitrification catalyst for NOx-containing exhaust gas for a long period of time, catalysts whose activity has decreased often have dust deposited on the catalyst surface, which must be removed by cleaning with water or a dilute inorganic acid aqueous solution in advance. However, at this time, water-soluble poisonous substances such as alkali metal compounds are also removed. On the other hand, since some of the active ingredient vanadium and the above-mentioned α component are eluted, prolonged washing is not preferred. In addition, if the contamination by dust or poisonous substances is significant and it is difficult to remove them only by cleaning with water or a dilute inorganic acid aqueous solution, it is effective to further clean with an oxalic acid aqueous solution, preferably under heating. The catalyst from which dust and poisonous substances have been removed by the above washing is a vanadium compound or [vanadium compound +
The slurry is applied by a method such as immersion in a titania slurry containing [alpha component compound]. The viscosity of the slurry is preferably 0.3 to 2.0 poise, and if it is less than 0.3 poise, the activity will not be recovered sufficiently;
Moreover, if it is more than 2.0 poise, the thickness of the coating layer increases, which is not preferable because it causes an increase in ventilation pressure loss, cracking of the coating layer, and promotion of peeling. The catalyst coated with the slurry is dried and calcined to be regenerated into a catalyst having activity and aeration pressure loss comparable to those of unused catalyst. As detailed above, a coated vanadium-titania catalyst whose activity has been reduced by the method of the present invention,
That is, V ₂ O ₅ −TiO ₂ and (V ₂ O ₅ + α component) −
A catalyst made of a combination of TiO ₂ can be regenerated to the same level as an unused catalyst without losing its shape, so its practical value is extremely high. Next, the present invention will be explained in more detail with reference to Examples. Example 1 When boiler exhaust gas was treated for two years under the following conditions using a coated honeycomb catalyst containing 91wt% TiO ₂ and 9wt% TiO ₂ , the initial NOx removal rate of 83% at 380°C decreased to 74.5%. . Exhaust gas treatment conditions Processing gas amount Temperature Space velocity 100Nm ³ /h 300 to 400℃ 10000 1/h NH ₃ /NOx ratio. NOx SOx 1.0 350ppm 1000ppm This catalyst with reduced performance is immersed in water and hot water that is 3.3 times the apparent volume of the catalyst (calculated from the external dimensions).
Washing was carried out for 30 minutes to 1 hour while circulating water and warm water. After washing, the catalyst was colored orange due to the elution of vanadium, and when the amount of vanadium in the washing water was determined by chemical analysis, it was found that 5 to 20% of the vanadium in the catalyst had been eluted. Also, potassium under all conditions
It was 100% eluted. Comparative Example 1 was obtained by drying the washed catalyst and measuring the NOx removal rate under the same exhaust gas treatment conditions as above.
As shown in Table 1, there was no complete recovery. The catalyst was further washed in hot water at 70°C for 30 minutes in the same manner as above, dried, immersed in an aqueous solution of methylamine in which ammonium metavanadate was dissolved, and subjected to exhaust gas treatment. As shown, it was 77.5%, almost the same as the one that was washed with water only. Next, a catalyst was obtained which was washed with water and warm water for 30 minutes to 1 hour in the same manner as in Comparative Example 1, and the proportions of TiO ₂ 88.7 wt% and ammonium metavanadate 11.3 wt% were weighed, and the viscosity was adjusted by adding an aqueous methylamine solution.
The slurry was applied by immersion in 0.5 poise titania slurry, dried and fired, and the NOx removal rate was measured under the exhaust gas treatment conditions as shown in Table 1. As shown in Table 1, the NOx removal rate was completely recovered to the initial NOx removal rate.

[Table] Example 2 A catalyst with degraded performance obtained in the same manner as in Example 1 was first lightly washed with water at room temperature, and then an aqueous oxalic acid solution (70°C) with an amount of 3.3 times the apparent volume of the catalyst was washed.
and washed for 1 hour while circulating the liquid. After washing, the color turned blue due to the formation of vanadyl oxalate. The elution rate of vanadium from the catalyst at this time varied as shown in Figure 2 depending on the oxalic acid concentration. 25~50g
Approximately 90% of vanadium is eluted at the concentration of (oxalic acid dihydrate)/(H ₂ O), and the elution rate does not improve even if the concentration is further increased. It was observed that the titania coated on the honeycomb substrate was also considerably peeled off during cleaning with an oxalic acid aqueous solution. Example 1: Drying the catalyst washed with oxalic acid aqueous solution
When the NOx removal rate was measured under the same exhaust gas treatment conditions as in Comparative Example 3 (Table 2), it was a very low value. Further, when the catalyst washed with an oxalic acid aqueous solution was dried, immersed in a vanadyl oxalate aqueous solution, dried, fired, and subjected to exhaust gas treatment, no effect was observed as shown in Comparative Example 4. Next, the catalyst was obtained by washing with an oxalic acid aqueous solution.
The slurry was applied by immersing it in titania slurry with a viscosity of 1.0 poise, which was adjusted by adding weighed water at a ratio of 85.5 wt% TiO ₂ and 14.4 wt% vanadyl oxalate.
After drying and firing, the NOx removal rate was measured under the same exhaust gas treatment conditions as in Example 1, and as shown in Table 2, the NOx removal rate recovered to the initial NOx removal rate.

[Table] Example 3 Boiler exhaust gas was treated for two years in the same manner as in Example 1 using a coated honeycomb catalyst containing 91wt% TiO ₂ , 2wt% V ₂ O ₅ , and 7wt% WO ₃ . The removal rate dropped from 84% to 76%. Gently wash the catalyst whose performance has deteriorated by pouring room-temperature water over it and use hot water (70°C) that is 3.3 times the apparent volume of the catalyst.
Alternatively, it was immersed in an oxalic acid aqueous solution (70°C) and washed for a certain period of time while circulating the solution. At this time, the elution rate of vanadium and tungsten from the catalyst changed as shown in FIG. Obtain the cleaned catalyst as described above, TiO ₂ 89.7wt
%, ammonium metavanadate 2.5 wt%, and ammonium paratungstate 7.8 wt%. The samples were dipped in titania slurry of various viscosities prepared by adding methylamine aqueous solution to coat the slurry, then dried and fired. The NOx removal rate and catalyst packed bed pressure loss were measured under the same exhaust gas treatment conditions as in Example 1, and the results shown in FIGS. 4 and 5 were obtained. If the slurry viscosity exceeds 2.0 poise, the pressure drop will rise rapidly, and if the slurry viscosity becomes 0.3 or less,
The NOx removal rate is lower than 80%, which is inappropriate. Note that the pressure loss of the unused catalyst was 28 mmH ₂ O/m. Example 4 A catalyst with degraded performance obtained in the same manner as in Example 1 was immersed in a 1% aqueous sulfuric acid solution (50° C.) in an amount 3.3 times the apparent volume of the catalyst, and washed for 1 hour while circulating the solution. After that, it was immersed in the same titania slurry (viscosity 0.5 poise) as in Example 1, the slurry was applied, dried, and fired, and the NOx removal rate was measured under the same exhaust gas treatment conditions as in Example 1. 83.5
It was %. Example 5 Boiler exhaust gas was treated for two years under the same conditions as Example 1 using a coated honeycomb catalyst containing vanadium and tungsten as catalyst components.
Initial NOx removal rate of 84% at 380℃ is 76%
It declined to . A catalyst with degraded performance obtained in the same manner as in Example 3 was washed with water at room temperature, and further immersed in an aqueous solution of oxalic acid (concentration 25 g/) with an amount of 3.3 times the apparent volume of the catalyst, and heated while circulating the solution. Washed for hours. The elution of vanadium at this time varied depending on the temperature as shown in Table 3. Moreover, the exfoliation of titania also increased with the increase in temperature. This catalyst was immersed in titania slurry with a viscosity of 1.0 poise prepared in the same manner as in Example 3, the slurry was applied, and the NOx removal rate was measured under the same exhaust gas treatment conditions as in Example 1. However, as shown in Table 3, the NOx removal rate recovered to the initial level.

【table】 [Brief explanation of the drawing]

Figure 1 shows an example of the structure of a honeycomb-type catalyst that is generally used as a denitrification catalyst, and Figure 2 shows the oxalic acid concentration when cleaning a TiO ₂ -V ₂ O ₅ catalyst with degraded performance with an oxalic acid aqueous solution. This is a graph plotting the elution rate of vanadium. Figure 3 shows the elution rate of vanadium and tungsten corresponding to the concentration of oxalic acid when a TiO ₂ -V ₂ O ₅ -WO ₃ catalyst with degraded performance is washed with an oxalic acid aqueous solution. This is a graph plotting the elution rate, and Figures 4 and 5 show
2 is a graph plotting the NOx removal rate and pressure loss of the regenerated catalyst in response to the viscosity of the titania slurry used when regenerating the TiO ₂ --V ₂ O ₅ --WO ₃ catalyst.

Claims (1)

[Claims] 1 Used vanadium-titania-based material consisting of V ₂ O ₅ and TiO ₂ or with WO ₃ and/or MoO ₃ added thereto, whose activity has decreased due to adhesion or accumulation of dust or poisonous substances. When regenerating a coated denitrification catalyst, after washing the catalyst with water or a dilute inorganic acid aqueous solution, it may be left as is or further washed with an oxalic acid aqueous solution, and then a titania slurry containing a vanadium compound with a viscosity of 0.3 to 2.0 poise is applied. A method for regenerating a used denitrification catalyst, which comprises drying and calcination.